Preheat oven to 500°.

Place potatoes in a large saucepan; fill with water to 1 inch above potatoes. Bring to a boil; cook 10 minutes or until potatoes are tender. Drain; return to pan. Mash potatoes to desired consistency with a potato masher. Stir in milk, butter, 1/4 teaspoon salt, and 1/4 teaspoon pepper.

Combine remaining 1/2 teaspoon salt, remaining 1/2 teaspoon pepper, 1 teaspoon oil, thyme, and garlic; rub evenly over pork. Place pork on a jelly-roll pan coated with cooking spray; bake at 500° for 10 minutes. Combine remaining 1 teaspoon oil, jelly, vinegar, and shallots in a small saucepan; bring to a boil. Cook 1 minute, stirring occasionally. Brush half of jelly mixture over pork; bake at 500° for 5 minutes. Turn pork; brush with remaining half of jelly mixture. Bake 5 to 8 minutes or until a thermometer registers 145°. Place pork on a cutting board; let stand 5 minutes. Cut across the grain into thin slices. Serve with mashed potatoes.